FIG. 1 shows a schematic diagram of a typical fire alarm system. As FIG. 1 shows, in the fire alarm system 100, a fire alarm control apparatus (Control Panel) 110 is connected to multiple fire detectors 120, notification devices 130 or manual alarms 140 etc. distributed throughout a building. The fire detectors 120, notification devices 130 and manual alarms 140 may be collectively referred to as peripheral devices. These peripheral devices may be connected to the fire alarm control apparatus (Control Panel) 110 via a wired or wireless network, and may obtain electrical energy from a line connected to the fire alarm control apparatus. In the fire alarm system shown in FIG. 1, the notification device 130 may for example emit a sound alert audible to a human (e.g. by means of a buzzer or loudspeaker), and may also emit a visual alert which can be observed by a human (e.g. using flashing light (a strobe)). For example a visual alert using flashing light is especially suitable for alerting people with a hearing impairment, and is also especially suitable for the noisy environment of a large shopping mall for example.
In the prior art, a notification device uses a xenon lamp or light-emitting diode (LED) as a flash element. In order to trigger the action of the flash element, it is generally necessary to provide an energy storage element in the notification device, e.g. a capacitor. The energy storage element can store a high level of electric energy, in order to provide the high voltage needed to trigger the xenon lamp or LED. FIG. 2 shows a circuit block diagram of a common notification device with an LED as a flash element. In FIG. 2, the notification device 200 for example comprises an input end 210, a current-limiting circuit 220, a booster circuit 230, an energy storage element 240, a flash control circuit 250 and an LED 260 connected in sequence. Specifically, the booster circuit 230 boosts an input voltage Vin received by the input end 210, and charges the energy storage element 240 to the boosted voltage value Vcap. When charging is complete, the voltage across the energy storage element 240 is higher than the input voltage Vin of the input end. The energy stored in the energy storage element 240 is used to supply the LED 260 for flashing. The flash control circuit 250 controls when the LED flashes, when it stops flashing, and the flashing intensity. In general, the LED flashes periodically, e.g. at a flashing frequency of 1 Hz, with the duration of each flash being approximately a few tens of milliseconds.
Each time the LED flashes, a drive current flows through the LED under the control of the flash control circuit 250, causing the LED to light up. At the same time, since the flashing of the LED consumes electrical energy, the voltage across the energy storage element 240 gradually falls and reaches its lowest point, also called the residual voltage Vr, when the flash ends. In general, the residual voltage Vr across the energy storage element 240 in each flash operation or after the flash will be less than the input voltage Vin of the input end. This causes a large current to flow for a short time from the input end to the energy storage element. A large current within a short space of time is generally called an inrush current. Such inrush currents occurring during periodic flashing are called repetitive inrush currents. Considering factors such as the flashing frequency (maximum 1 Hz) and power losses, it is generally very difficult to control such a repetitive inrush current.
In view of this, a current-limiting circuit 220 is also provided in the example shown in FIG. 2. The current-limiting circuit 220 can sense, in real time, the size of current flowing to the energy storage element 240, and compare it with a fixed threshold. If the current at the present time exceeds the threshold, the booster circuit pauses operation, and waits for the current to fall back to below the threshold, and thereby suppresses the occurrence of a repetitive inrush current. U.S. Pat. No. 724,314 describes a structure of a current-limiting circuit with the abovementioned similar function.